a bit confused about apparent attenuation...

Discussion in 'General Brewing Discussions' started by Gledison, Nov 26, 2017.

  1. Gledison

    Gledison Active Member

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    hi everyone. hope the fellows in US had a great thanksgiving holiday full of new beers:)
    look, im getting a bit confused (perhaps overthinking) about the recommended yeast attenuations.
    I want to follow the tips from the folks here for my IPL. Some say to bring the fermentation to ca.68°F after 75% attenuation in order to do a diacetyl rest (in case needed). As im doing a very small batch, I cant take big samples, only small drops for the refractometer. The question is when brewers talk about 75% attenuation, it means based on all the sugars present (fermentable and unfermentables) or based only in the fermentables? I think the max attenuation of my Yeast (Gozdawa 18) is 72%. I belive that it considers all the gravity as fermentables but I would liket to check this. Im not sure either if sofwares can be based on mash profile in order to calculate the amount of potential fermentable sugars (being that the 100%).
    im sure you guys can help to clarify things
    Great sunday to all
     
  2. J A

    J A Well-Known Member

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    When it's 75% done with the fermenting process. In other words 75% of the difference between OG and (expected) FG. If you're going from 1.050 to 1.010, for instance, you be looking for 40 points so when it's gone 30 points to 1.020, that's when you'd start raising temp. Truth is that if you raise for D-rest any time before FG is reached and the yeast start to settle, it'll probably clean up diacetyl.
     
  3. Gledison

    Gledison Active Member

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    Got it.
    the OG value is based on the theoretical ppg for each malt right, and the FG based on the maximum yeast attenuation. right?
     
  4. Nosybear

    Nosybear Well-Known Member

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    That sounds about right. If my malt will give me a theoretical yield of 32 ppg (normal for a base malt), and my efficiency is 80%, I'll get about 26 ppg from the grain (32 * 0.8 = 25.5). If I multiply that times the pounds of grain, let's use 10 for ease of arithmetic, I'll get 260 gravity points. If I then divide by 5 gallons, the batch size, I get 51.2 for an original gravity of 1.051, rounded (divide the result by 1,000 and add 1). If my yeast has an apparent attenuation of 75%, the number reported for attenuation, I multiply the 51.2 * (1-0.75) to get a final gravity of 12.8, specific gravity conversion as above to get 1.0128, rounded to 1.013. In brewing, we rarely use the "real" attenuation, we just let the alcohol volume affect the final figure to get apparent attenuation.

    You'll use points per kilogram per liter in metric but the math is the same.
     
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  5. J A

    J A Well-Known Member

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    OG is gravity of the wort as measured pre-pitch. The projected OG is based on malt and efficiency, but you should have a measurement of the OG after the mashing/boiling process is done. Without that all the potential calculations don't do you much good in the real world. Mash efficiency will have an affect on OG and mash temperature and alpha vs beta conversion will have an effect on the ability of the yeast to use the sugars produced during mashing and thereby will impact potential attenuation.
    As long as you have mashed enough times to have a good idea of your system's efficiency and you're pretty accurate about using the mash temps and times that you set up in the recipe calculator, you can count on hitting the numbers pretty close. But the only measurements that really matter in terms of how the beer actually turns out are OG and FG as measured with a hydrometer pre- and post fermentation. All the numbers you thought you might get based on the recipe calculator are irrelevant at that point and the beer is what it is.
     
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  6. Nosybear

    Nosybear Well-Known Member

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    ^^^^ What he said. To be clear, I wasn't talking about taking an OG reading, you should do that every time. I was talking about how to calculate the projected OG value. As JA said, some of the variables will be different for each system and process and the software does have to make some assumptions. You change those by changing your profile. I generally hit around 80% conversion efficiency for single infusion mashes, around 85% for step mashes but your numbers will likely be different.
     
  7. Gledison

    Gledison Active Member

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    yep, i totally agree with you. I just want to be sure im thinking right. im always measuring all the values but since I've got a refractometer as well, i thinking im getting to many data:p
    OG and FG always measured with hydrometer. the big question was just about if the mash profile in calculators will "tell me" the expected final gravity based on that mashing profile, instead of giving me the FG considering that all sugars in my wort are fermentable
     
  8. J A

    J A Well-Known Member

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    Yes...set a mash rest for 45 minutes and change the temp through a range of temperatures and notice the difference in FG.
     
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